Injection molded articles with improved surface characteristics, production of same and apparatus therefor

ABSTRACT

In injection molding thermoplastic resin compositions containing reinforcing materials and/or fillers, high quality molded articles superior in surface gloss and exhibiting substantially no surface defect can be obtained, by employing a technical measure, in which only superficial layer of the inner wall surface of the mold is selectively heated, by high-frequency induction heating, to a temperature above the heat distortion temperature of the thermoplastic resin composition employed prior to injection molding and cooling and solidifying the resin.

FIELD OF THE INVENTION

The present invention relates to injection molded articles ofthermoplastic resin compositions containing reinforcing materials and/orfillers, and which exhibit improved appearance, especially in surfacegloss. The invention also relates to a method of production of suchinjection molded articles and to the apparatus used in the saidproduction.

DESCRIPTION OF THE PRIOR ART

Heretofore, injection molding of thermoplastic resin has, in general,been based on a technique, in which a resin mixture is molded in a metalmold by utilizing the plasticity of the thermoplastic resin, that is tosay, by rendering the thermoplastic resin followable by heating it in,such as, screw, etc., and is then solidified in the mold by cooling itto obtain the molded article. Thus, it is necessary to cool the moldedresin mixture to a temperature below the heat distortion temperature ofthe resin employed, in order to attain solidification of the resinmixture, so as to release and remove the molded article from the mold ina satisfactory manner. For this reason, the temperature of the metalmold is held usually below the heat distortion temperature of the resinemployed. Also it has currently been practiced to cool the metal mold toa temperature closely above the dew point by using a refrigerant, inorder to increase the productivity. Even when, in cooling the metalmold, the sensible heat of the molten resin is used for, such as,heating and regenerating, the temperature of the metal mold inaccordance with the principle of the technique, should be kept below theheat distortion temperature of the thermoplastic resin. The moltenthermoplastic resin mixture, upon contact with the cold surface of themetal mold, is cooled abruptly and loses its flowability sharply nearthe surface of the mold, whereby the fitness onto the mold surface isgreatly impaired to result in a considerable irregularity on the surfaceof the molded article.

When reinforcing materials and/or fillers are employed, withcompatibility of the reinforcing substances and/or of fillers with thethermoplastic resin being, in general, low, micronous interspace overthe interface between the particle surface of the reinforcing materialsand the thermoplastic resin may be formed, causing so-called silverstreak to appear on injection molded articles. Thus, only moldedarticles of poor appearance with silver streaks, surface irregularitydue to exposure of the reinforcing materials and/or fillers on the outerface, can be obtained.

As explained above, in producing molded articles from thermoplasticresin compositions, especially those containing reinforcing materialsand/or fillers by injection molding, it is important to preventsolidification of the molten composition by cooling while it is flowingwithin the mold cavity.

As a measure for preventing such peripheral solidification of the resinmixture, it has been proposed to elevate the temperature of the metalmold. However, an increase of the temperature of metal mold willnaturally require a longer cooling time, and this will result in moldedarticle being taken out of the mold, while still incompletely solidifiedand thus exhibiting poor dimensional stability. Therefore, in actualpractice, the temperature of the metal mold is adjusted at a temperaturecompromising the adverse effects of these contradictory conditions.

Explaining one typical example of the conventional art, there isproposed in Japanese published examined patent application No.22,020/1970, a technical measure, in which the inner surfaces of themetal mold are pre-heated superficially by introducing a hightemperature fluid into the mold cavity prior to the injection of moltenresin mixture thereinto. This method however, brings forth difficultiesin that, since the fluid is heated after it is in the mold, the residualfluid may cause various streaks and spoil over the surface of the moldedarticles. And, since in some cases, heated fluid is introduced into themold, inhomogenerous heating of the mold inner surface may occur atportions where the mold cavity exhibits a projection or a recess, suchas, cause for a rib or a stub resulting sink marks, irregular gloss andso on of the molded articles. In extreme cases, the resin may evenadhere to the mold surface making it difficult to remove therefrom. Thiscan even result in the molded article to be broken at time of removaland thus molded articles of satisfactory appearance cannot be expected.

The surface gloss is a very important feature that determines themarketability of the injection molded article. The surface glosscorresponds to a reproductivity of the smooth surface of the metal mold,which in identical molding conditions depends on the degree of finish ofthe mold inner surface. Thus, surface gloss is best, when a mold havingperfect mirror surface is used. However, this resulting best surfacegloss depends, on the other hand, upon the composition of the resinmixture. In general, this surface gloss decreases with increase of thecontent of additives such as reinforcing materials and/or fillers. Inparticular, an injection molded article of a resin compositioncontaining additives in an amount adequate to impart sufficientreinforcement or filling effect will exhibit under ordinary conditionsof injection molding poor gloss which is considerably inferior than thatof the molded article of resins having no additives.

SUMMARY OF THE INVENTION

One object of the present invention is therefore to provide injectionmolded articles of thermoplastic resin compositions containingreinforcing materials and/or fillers, said molded articles exhibitingsurface gloss comparable to those of injection molded resin articleswithout additives and which exhibit no surface defect such as silverstreak etc.

Another object of the present invention is to provide an improved methodof injection molding which permits the manufacture of injection moldedarticles exhibiting superior surface characteristics mentioned abovefrom thermoplastic resin compositions containing reinforcing materialsand/or fillers, in a shorter molding cycle.

A further object of the invention is to provide an injection moldingapparatus suitable for realizing the above identified improved method ofinjection molding.

According to the present invention, there are provided injection moldedarticles of thermoplastic resin compositions containing reinforcingmaterials and/or fillers in an amount at least 4% by weight, comprisingsmooth skin layer constituted substantially of only the resin componentover the outer surface of the molded article, said layer bestowing onthe article a mirror surface having surface gloss (ASTM D 523; 60°) wellmaintained within a decrement of reflectivity of 10% or less, preferablybelow 5%, and more preferably 3% or less, based on the perfect mirrorreflection of the individual resin.

The injection molded articles according to the present invention can beproduced by a method of injection molding thermoplastic resincompositions containing at least 4% by weight of reinforcing materialsand/or fillers, comprising, selectively pre-heating by a high-frequencyinduction heating, the inner surface of the metal mold superficially toa temperature above the heat distortion temperature of the resinemployed, before the injection of the molten resin mixture into themold, so as to permit sufficient flow of the molten resin mixture incontact with the inner surface of the metal mold to form a smooth skinlayer with a thickness of preferably 1-100μ consisting substantially ofonly the resin component.

The present invention provides further an injection molding apparatus tobe adopted suitably for performing the above mentioned method ofinjection molding of thermoplastic resin mixture, said apparatusconsisting of (a) an injection arrangement having means for melting,metering and injecting the resin mixture and (b) a metal mold providedwith means for cooling and solidifying the molded resin mixture,comprising

(c) a high-frequency oscillator and

(d) a high-frequency induction heating device constituted of an inductorwhich is connected to said oscillator and is disposed in theneighborhood of said metal mold so as to enable to heat the innersurface of the metal mold only superficially.

The characteristic feature of the present invention resides in that theinner surface of the metal mold is selectively heated only superficiallyto a temperature above the heat distortion temperature of the resinusing a high-frequency induction heating, whereby a smooth mirrorsurface with increased gloss is brought about on the molded resinarticle. The heat distortion temperature herein used corresponds to thatprescribed by ASTM D 648 (18.6 Kg/cm² Fiber Stress). By the words"selectively heat the surface superficially", it is meant, that theinner surface of the metal mold is heated instantaneously only to adepth of a skin layer using high-frequency induction heating.

Such instantaneous heating can only be achieved by a special heatingmethod of high-frequency induction heating. It is essential forattaining the objects of the present invention, that the temperature inthe skin layer of the mold inner surface is elevated at a fast rate. Theactual rate of heat elevation is determined by taking into account ofthe actual heat distortion temperature of the resin employed, size ofthe molded product, the mold releasing temperature which is determinedsuitably in accordance with the foregoing factors, and so on for eachresin. It is recommended, however, to heat to a predeterminedtemperature at a heat elevation rate of 80° C. per minute or more,preferably 480° C./min. or higher and most preferably at least 1200°C./min. By employing such instantaneous heating, only a thin layer overthe inner surface of the metal mold can be heated above the heatdistortion temperature of the resin without the heat being conductedinto the interior of the mold metal and without causing the whole metalmold to be heated, so as to accommodate to the prompt heat removal attime of cooling. Thus, it is possible to shorten the molding cycle withsimultaneous attainment of higher surface quality of the moldedarticles. Furthermore, by employing the high-frequency inductionheating, such things that may cause to contaminate the metal mold as theheating fluid mentioned previously is completely excluded, thuseliminating the possible deterioration of the molded product. Otheradvantages of the use of high-frequency induction heating may be recitedas follows:

(a) facilitated temperature control,

(b) enabling either homogeneous heating over the whole surface of themold or selective heating including local heating of specific areabeside the above mentioned superficial heating, thus enablingvoluntarily to heat either of the whole mold or of local part of themold,

(c) eliminating adverse effects of heat on the operators

(d) offers a push-button automatic operation.

The invention is further explained below with reference to the drawingsappended, so as to facilitate the understanding of the principle of thepresent invention.

FIG. 1 shows an embodiment of the apparatus to be employed forperforming the method of injection molding according to the presentinvention in schematic illustration.

FIG. 2 is another embodiment comparable to FIG. 1.

FIG. 3 shows only the portion of metal mold employing a high-frequencyinductor pendingly inserted within the mold cavity, in vertical section.

FIG. 4 shows another embodiment of inductor of built-in type also invertical section.

FIG. 5 is a graph showing a typical temperature distribution within themold.

FIG. 6 illustrates the correlation between the gloss (20°) in % and thegloss (60°) in % for molded articles according to the present invention.

FIG. 7 illustrates the size and shape of the specimen used in Examples.

FIG. 8 is an explanatory illustration of weld line on the molded productin lateral section.

FIG. 9 is a microphotograph (X7000) of a cross section of a shapedarticle employing high impact polystyrene according to the presentprocess.

FIG. 10 is a microphotograph of the similar article to FIG. 9, accordingto conventional process.

FIG. 11 is a microphotograph (X440) of a cross-section of a shapedarticle employing glass fiber reinforced styrene-acrylonitrile resin(herein referred to "SAN-GF") according to the present process.

FIG. 12 is a microphotograph of the similar article to FIG. 11,according to conventional process.

DETAILED DESCRIPTION OF THE INVENTION

As shown in FIGS. 1 and 2, the apparatus according to the presentinvention consists of an injection molding machine and a high-frequencyinduction heating device. The high frequency induction heating device iscomposed of a high-frequency oscillator 1 and an inductance coil(inductor) 2 installed near the inner surface of the metal mold andconnected to the oscillator 1. The injection molding machine is composedof an injection cylinder segment 3 for realizing the melting andinjecting of the resin mixture and a mold segment consisting of astationary split mold half 4 and a movable split mold half 5. In theembodiment shown in FIGS. 1 and 3, the inductor is inserted in the moldcavity by being pinched between the two mold halves of the split metalmold. In the embodiment shown in FIGS. 2 and 4, the inductor 2 is builtin internally of the mold.

In FIG. 3, the mold segment and the inductor of FIG. 1 are shown in anenlarged view. The inductor 2 for the high-frequency induction heatingis placed between the stationary mold half 4 and the moving mold half 5.When it is energized by a high frequency oscillation, it can berecognized that only the temperature in the surface layer of the metalmold (at points A and B) is increased steeply and the temperature in thebulk of the mold (at points C and D) is kept almost unchanged, as shownin FIG. 5. The temperature-time diagram shown in FIG. 5 illustratesexemplarily the course of temperature changes at various portions of themetal mold after high-frequency induction heating without employingwater-cooling of the mold. The split metal mold is once opened, when thetemperature of the mold surface has reached a predetermined temperature.The inductor 2 is withdrawn from the space between the fixed mold half 4and the movable mold half 5. Subsequently, the split mold is closedagain to carry out the injection molding of thermoplastic resin mixturein a conventional manner.

FIG. 4 shows another performance form of the apparatus according to thepresent invention in which the inductor is arranged in the metal mold asa built-in installation. The mold shown is a two-sectioned center-directgate type split mold for manufacturing dish-like article having adiameter of about 10 cm. The portions A and A' constitute the moldcavity (that determines the shape and appearance of the molded article)and are made of the usual mold metal, SC metal such as S-45C, S-55C,etc., plate metal thereof, ultra hard mold metal (alloyed tool steel) ora mold steel such as NAK, SKD 11 or the like. The symbols B and B'denote the inductor for high-frequency induction heating. The inductoris prepared in such a manner, that a copper tube is wound into spiraland is consolidated by embedding in a hardened resin such as epoxy etc.

C and C' indicate insulation layers for high-frequency wave made from anon-magnetic metal, as will be explained afterwards in detail. D and D'represent the matrix, in which other functional mechanisms requisite forinjection molding are embedded. This matrix is furnished with, such as,guide pin, flange and holes for fixing the metal mold, studs and so on.For the material of the matrix, any mold metal can be employed. Anordinary steel, for example, an SC steel, such as S-45C or S-55C may berecommended for its durability. The cooling water may be circulatedthrough perforations arranged either in the matrix or, in order toupgrade efficiency, in the portions A and A'.

It may be possible in accordance with the configuration of the moldedarticle, to unite or intergrate the portions C, C' with D, D' so as touse only one and the same material, for example, Be-Cu or so on.

In case the inductor is merely embedded in the mold metal, portions ofmold metal close to the inductor will be heated, and thus, causing theinterior of the mold metal to be heated. Due to such useless heatinginside the mold metal, the oscillator tends to be subject to overload,which may cause an actuation of the overload breaker, resulting in acessation of also the necessary superficial heating.

The inventors having taken notice of the fact that there is aselectivity among materials in their susceptibility to high-frequencyinduction heating, studied various materials and found it best to usenon-magnetic metals. In consequence of the study, it was found thatsuccessful results were obtainable, when the portion of the metal moldexposed to the molten resin mixture and to be heated is constituted of amaterial subject to the high-frequency induction heating, such as aferrous metal containing predominant amount of iron such as steel, forexample, S-45C, S-55C, NAK or so on, and the portion of the metal moldrequiring no heating is constituted of a non-magnetic metal.

Here, in general, non-magnetic metals other than Be-Cu alloy are softand are not suited well for the matrix metal with regard to theirdurability.

The inventors had therefore made their efforts to direct to overcomesuch circumstances and have found, that the portion of the metal moldwhere no heating is required can be isolated from the high-frequencywave, when a thin layer of a non-magnetic metal is interposed betweenthe inductor and said metal mold portion.

A thickness of this non-magnetic metal layer of 0.5 mm or more willoffer a sufficient insulation for the purpose of the present invention,whereas a thickness below 0.5 mm may bring forth faulty result. Forinstance, an aluminum foil of 0.1 mm thickness will fuse down upon thehigh-frequency induction heating and will not build up an insulatinglayer.

Thus, an injection molding process and apparatus thereof comprising ametal mold equipped with embedded inductor for heating selectivity thesuperficial layer of the mold inner surface contacting the injectedresin mixture according to the present invention, in which an insulatinglayer against the high-frequency wave is interposed between the inductorand the portion of the metal mold requiring no heating, and whichenables steep heating and cooling utilizing high frequency inductionheating, is most effective.

The non-magnetic metal to be used according to the present inventionincludes Cu, Al, Be and alloys consisting predominantly of these metalsinclusive of bronze, beryllium copper and so on. Ceramics, glass, woodand the like are also non-magnetic substances, but these are notsuitable for use as material for molds for their inferior heatconductance, durability and so on.

The injection molded articles according to the present inventionacquires from the superficially heated inner surface of the metal mold acorresponding reproductive surface with excellent surface gloss. Thegloss is well kept within a range in which the decrement of reflectivityfrom the ideal perfect mirror reflection is at the most 10%, based onthe perfect mirror reflection of the employed resin itself.

Here, by the perfect mirror reflection of the resin is meant anintrinsic measure of gloss determined in relation to the index ofrefraction of each individual resin employed, which corresponds to apercentage indication of the reflectivity measured at a prescribedstandard angle of incidence (60° in the specification of the presentinvention) relative to the reflectivity of smooth surface of a glasshaving an index of refraction of 1.567, measured at the same standardangle of incidence, as is instructed by JIS Z 8741. The perfect mirrorreflection for each index of refraction of resin can be seen from thefollowing table:

    ______________________________________                                                      Gloss at Angle of Incidence                                                   of 60° for Perfect Mirror                                Index of Refraction                                                                         Reflection of Resin in %                                        of Resin      (Gs 60°)%)                                               ______________________________________                                        1.500         89.1                                                            1.520         92.4                                                            1.540         95.7                                                            1.560         98.9                                                            1.580         102.1                                                           1.600         105.1                                                           ______________________________________                                    

A perfect mirror reflection of any voluntary resin can be estimated fromthe above table by interpolating from the given values. For instance, ina so-called high impact polystyrene (HIPS) reinforced by 5-20% by weightof a rubber (e.g. polybutadiene), the "individual resin employed" hereis polystyrene. Index of refraction n of a polystyrene lies in generalat about 1.592 and the perfect mirror reflection is calculated to be104.6. A high impact polystyrene moled article obtained according to thepresent invention exhibits therefore a surface gloss value (Gs (60°)%)of from about 94.6% (decrement of 10%) to about 104.6%.

In another example of resin of an ABS copolymer with monomer ratioacrylonitrile/styrene of 30/70 and containing about 5 to 35% by weightof butadiene, the individual resin employed is the acrylonitrile/styrenecopolymer with mirror ratio 30/70. Here, the index of refraction nequals to 1.577 and the perfect mirror reflection calculates to 101.6%.For the case where the acrylonitrile/styrene ratio corresponds to 25/75,the refractive index n equals to 1.579, corresponding to a perfectmirror reflection value of 101.9%. Thus, if ABS resins exhibiting A/Sratios of 30/70 and 25/75 are employed, the surface gloss values (Gs(60°)%) of the injection molded articles obtained fall within the rangeof from 91.6 to 101.6% and from 91.9 to 101.9% respectively. Likewise,injection molded articles of polyphenyleneether (PPE) resins accordingto the present invention show surface gloss values (Gs 60°) in a rangefrom about 80 to 108%, especially from 90 to 108%, depending also on theindividual PPE used.

In this specification, the evaluation of the surface gloss of moldedarticle is based on the value of Gs(60°)% of ASTM D 523, in accordancewith the current practice for evaluating molded articles of plasticmaterials for their appearance and glaze, wherein a % indication ofgloss at insident angle of 60° is employed for the evaluation. Followingstrictly the instruction of ASTM D 523, it is prescribed to employ aGs(20°)% value measured at an incident angle of 20° for estimating thegloss, when the Gs(60°)% value exceeds 70%. In this regard, measurementswere performed for estimating Gs(60°)% and Gs(20°)% values on moldedarticles according to the present invention, in order to clear thecorrelation between them. The results are shown in FIG. 6. If theinstruction of ASTM D 523 is followed exactly, the gloss of a moldedarticle showing Gs(60°)% value greater than 70% must be expressed by thecorresponding Gs(20°)% value obtainable from FIG. 6. In thisspecification however, the evaluation is performed exclusively byGs(60°)% value, following the conventional practice in the art since thedifference in relation to the Gs(20°)% is apparent.

Injection molded articles according to the present invention exhibit thesurface gloss intrinsic of the employed thermoplastic resin itself andshow no noticeably faulty appearance, such as, so-called flow mark,jetting, weld line and silver streak, caused from the irregular flow ofthe thermoplastic resin mixture containing reinforcing material and/orfiller. In the injection molded articles according to the presentinvention, it is preferable even for articles of complicatedconfigurations, such as lattice and the like, not to mention articles ofsimpler shapes, to have the said thermoplastic resin skin-layer ofthickness 1-100μ. The excellent surface gloss may be attributed to thefact that the molten resin mixture injected into the mold cavity ispermitted to keep its flow even on the surface of the metal mold, due tothe preliminarily conducted superficial heating of the metal mold, sothat a smooth skin layer constituted substantially of only the resincomponent without reinforcing material or filler may be formed over theinner surface of the metal mold under the filling up of dents and pitsdue to the reinforcing material or filler.

In a usual injection molding, weld line appears at the junction of flowof the molten resin mixture inside the mold as a conflux line in a formof thin groove having a depth of 3-5μ or more and a width of over 10μ,as shown in FIG. 8, in a lateral section. In injection molded articlesaccording to the present invention, substantially no noticeable weldline indention having depth less than 1μ and width less than 5μ occurs.

Flow mark can be formed by, for example, a disturbance of flow of themolten resin mixture and an irregularity in pressure transmission at aportion of varying wall thickness of the molded article, due to coolingand solidification of the molten resin mixture on the metal mold.Injection molded articles according to the present invention shows nosuch flow mark.

Silver streak appears on the surface of molded articles upon thesolidification of the resin mixture in a form of streak of silver colorwhile volatile materials etc. in the resin mixture are beingvolatilized. In injection molded articles according to the presentinvention, no silver streak can be found.

Jetting occurs often at the gate of the mold as a trace of partiallyprojecting line in the mold due to the acceleration of the flow of resinmixture through a narrow throat. This is also excluded in moldedarticles according to the invention.

All the deteriorations in the appearance mentioned above are based onthe irregularity in the flow of molten resin mixture inside the metalmold, and hence, are avoided in the molded articles according to thepresent invention by the improvement of the flow of resin mixture on themold surface.

According to the present invention, there occurs no solidification ofthe resin mixture by cooling upon entrance thereof into the metal moldand a uniform flow of the molten resin mixture over the whole innersurface of the metal mold can be warranted, since the inner surface ofthe metal mold has been heated preliminarily above the heat distortiontemperature of the resin. This results in a uniform surface gloss withsubstantially the same gloss regardless of the portion of the mold, saythe gate end or the dead end thereof.

Expressing the difference in the gloss of the molded article per unitlength along the line from the gate end to the dead end of the mold as"gloss gradient", this gradient is very low and amounts to 0-0.5,preferably to 0-0.2 and most preferably to 0-0.1%/cm for injectionmolded articles according to the present invention. In contrast thereto,the gloss gradient lay in most cases in the range from 1 to 5%/cm, forthe conventional injection molded articles having a ratio of resin flowlength L to the thickness t of the molded article L/t=20-30 or more.This shows the remarkable excellency of the molded articles according tothe present invention in their gloss and gloss irregularity.

While, by the method according to the present invention, it is possibleto obtain injection molded articles exhibiting superior surface glossfrom thermoplastic resin mixtures containing 4% by weight or more ofreinforcing materials and/or fillers, the method according to thepresent invention brings about injection molded articles showing no suchsurface defects, as silver streak, jetting, weld line etc., the same istrue when resin mixture containing less than 4% by weight of reinforcingmaterials and/or fillers or individual resin is used.

As for the thermoplastic resin capable of being employed in the presentinvention, those based on styrene and on polyphenyleneether are to berecited as typical examples therefor.

By the resins based on styrene are meant all the resins containingstyrene as predominant monomer component together with other subsidiarycomonomer components and/or reinforcing component. Concretely, thefollowings may be enumerated:

Polystyrene, acrylonitrile/styrene resin (AS resin), rubber reinforcedstyrene base resins such as HIPS and MIPS, butyl acrylaterubber/acrylonitrile/styrene copolymer (AAS), ethylene-propylenerubber/acrylonitrile/styrene copolymer (AES), ABS resins includingacrylonitrile/butadiene/styrene copolymer,acrylonitrile/butadiene/styrene/α-methyl styrene copolymer andacrylonitrile/methyl methacrylate/butadiene/styrene copolymer, and soon.

Among these resins based on styrene, those which show marked effectaccording to the present invention are the afore-mentioned rubberreinforced styrene base resins. These resins tend to offer rough surfaceupon injection molding, due to the large rubber particles contained,which are subjected to deformation upon movement inside the metal mold.Therefore, according to the conventional practice of injection molding,only gloss values around 60% are reached by rubber reinforced styrenebase resins containing 4% by weight or more of the rubber component. Incontrast thereto, it is possible according to the method of the presentinvention, to attain gloss values of about 90-100% by the same resins.By employing ABS resins, it is possible, according to the presentinvention, to obtain higher gloss values which are usually above 94%.

The resins based on polyphenyleneether to be used according to thepresent invention are those containing as the principal component (morethan 80%) a polyphenyleneether expressed by the general formula ##STR1##wherein R₁ and R₂ denote each an alkyl group having 1-4 carbon atoms andn represents the polymerisation degree, a polyphenyleneethergraft-copolymerized with styrenic compound or a resin mixture consistingof 20-80% by weight of one of these polyphenyleneethers and 80-20% byweight of a polymer based on styrene.

The thermoplastic resin composition to be used according to the presentinvention can contain other additives which are commonly employed in theart, such as, flame resisting agent, lubricating agent and so on.

Examples of the polyphenyleneethers expressed by the above generalformula are: poly(2,6-dimethylphenylene-1,4-ether),poly(2,6-diethylphenylene-1,4-ether),poly(2-methyl-6-ethylphenylene-1,4-ether),poly(2-methyl-6-propylphenylene-1,4-ether),poly(2-ethyl-6-propylphenylene-1,4-ether),poly(2-methyl-6-butylphenylene-1,4-ether),poly(2-ethyl-6-butylphenylene-1,4-ether) and so on.

The styrenic compound in the polyphenyleneether havinggraft-copolymerised a styrenic compound as mentioned above includesstyrene and its derivatives such as alkylated styrene, halogenatedstyrene and so on. Example therefor are: styrene, α-methyl-styrene,2,4-dimethyl-styrene, monochlorostyrene, dichlorostyrene,p-methyl-styrene, ethyl-styrene and so on.

It is possible to use concurrently upon the polymerisation anothercopolymerisable vinyl compound such as, methyl methacrylate,acrylonitrile, methacrylonitrile, butyl acrylate and so on. It ispossible to graft two or more styrenic compounds concurrently.

The constituent components of the polymer based on styrene as previouslystated may be the same with the above compounds to be used concurrentlyupon the graft copolymerisation.

The polymer based on styrene includes, according to the presentinvention, also the so-called rubber reinforced resins, such as rubberreinforced polystyrene, acrylonitrile/butadiene/styrene copolymer resinand polystyrene resin containing EPDM rubber.

When polyphenyleneether resins as identified above are used, theinjection molded articles obtained according to the present inventionwill exhibit, in general, surface gloss value of more than 80%.

It is of course possible to use other injection molding resins such aspolyethylene, polypropylene, polycarbonate, polyoxymethylene, nylon andso on.

The filler to be incorporated in the thermoplastic resin compositionaccording to the present invention includes those of inorganic nature,for example glass fiber, glass beads, calcium carbonate, mica, asbestos,and so on and powder and hollow material of metals such as iron, copper,zinc and aluminum, as well as oxides and hydroxides of these metals,each having a predominant particle size of 5-mesh or below.

The total amount of filler in the resin composition according to thepresent invention falls in general within a range of from 5 to 70% byweight, based on the total composition.

The high-frequency oscillator which can be employed according to thepresent invention may be of electromotive generator, electron tube orthyristor inverter types. A frequency in the range from 50 Hz to 10 MHzcan be used, while a frequency of from 1 to 1000 KHz may be recommendedin practice. The power output of the high-frequency oscillator may be inthe range from 1 to 5000 KW, to be determined suitably in accordancewith the size of metal mold to be heated, the temperature contemplatedand the rate of temperature elevation intended.

Here, the heating power P through a high-frequency induction heating iscalculated by the equation. ##EQU1## in which P is the heating power, ais the radius of the induction coil, f is the frequency, μ_(s) denotesthe specific magnetic permeability, n indicates the number of coilwindings per meter, I represents the electric current in the coil and ρis the specific resistance of mold metal.

For instance, a pertinent power output of an oscillator, in which theinductor is made from copper tube of 5 mm diameter by winding it at aninterval of 5 mm into a swirl and the distance between the inner surfaceof the metal mold and the inductor set at 1 cm, lies within a range from0.1 to 10 KW per 1 cm² of the surface area of molded article, oncondition that the frequency used is 400 KHz and the temperature of themetal mold of S 45 C is elevated by 40°-50° C. from the startingtemperature of about 40°-90° C. within a heating time of 10-15 seconds,as in the ordinary injection molding. At power outputs less than 0.1KW/cm², the rate of temperature elevation of the metal mold is too lowto be practical and eventually may suffer from overload, causing theoverload breaker to actuate and the heating to cease. When the poweroutput exceeds over 10 KW/cm², the rate of temperature elevation becomestoo steep to control the mold temperature and, in case of large metalmold with greater heating surface, a uniform heating becomes no longerpossible. If there is a temperature inequality of more than 50° C. overthe inner surface of the metal mold, gloss irregularities, sink marksand so on over the surface of the injection molded article will tend tooccur.

Hereinbelow, the invention is further explained in detail by recitingExamples, which however should offer no restriction on the scope of thepresent invention.

EXAMPLE 1

An AS resin composition containing 20% by weight of glass fiber having adiameter of 13μ was injection molded by an injection molding machine ofordinary in-line type shown in FIG. 1. The split mold made of anordinary steel of S 45 C was used and provided for molding a dish-likearticle having a diameter of 10 cm, a depth of 2 cm and average wallthickness of 3.5 mm. The mold is equipped with a center-direct gate.

Inductor was prepared by, having a copper tube with diameter of 3 mm waswound at an interval of 5 mm into a whirl and shaped to fit the profileof the mold cavity and then was consolidated by embedding it in an epoxyresin into a flat plate.

The temperature of the injection cylinder was adjusted so as to obtain aresin mixture temperature of 240° C. Before injecting the resin mixtureinto the mold, the inductor prepared as above was put between the twomold halves. After actuating the oscillation at 400 KHz, 6 KW, for 15seconds, the split mold was once opened to draw out the inductor beforeit was closed again. During this procedure, the cooling water was madenot to circulate in the mold metal. Then, the molten AS resincomposition containing glass fiber was injected into the mold at aninjection pressure of 60 Kg/cm² for 10 seconds, as in ordinary injectionmolding. Thereafter, cooling water was circulated in the mold metal for20 seconds to cool the molded article. Then, the molded article wastaken out of the mold. The molding cycle amounted to 60 seconds intotal.

The appearance of the molded article was excellent and was comparable tothat of molded article of AS resin only and showed no fault such assilver streak and exposure of glass fiber on the outer face. Gs(60°)% ofthe molded article was excellently high as much as 102%.

EXAMPLE 2

An ABS resin composition containing 20% by weight of glass fiber of adiameter of 15μ was molded by an ordinary type injection machine at aresin temperature of 240° C. The split mold employed was made of S55 Csteel and was so constructed that a dumnbell specimen having a shapeprescribed in JIS K 6871 and a rectangular plate specimen can be molded.Inductor was prepared by winding a copper pipe with diameter of 3 mminto whirl at an interval of 5 mm and embedding it in an epoxy resinplate of thickness of 2 cm. The procedure of injection molding was thesame as in Example 1, but with 400 KHz, 6 KW, 10 seconds ofhigh-frequency oscillation, 10 seconds of injection molding duration, 15seconds of water cooling, 50 seconds of total cycle time and 50 Kg/cm²of injection pressure.

The surface of the so molded article was covered by a skin layer of theABS resin employed. A molded article exhibiting beautiful appearance andsuperior gloss was obtained.

The molded article was evaluated for its properties according to theinstruction of JIS K 6871. The results were as given in Table 1.

As is seen from Table 1, the molded article obtained revealed anexcellent appearance, superior gloss and other favorable properties.

EXAMPLE 3

A PS resin composition containing 50% by weight of 200-mesh iron powderwas molded by an ordinary in-line type injection machine at a resintemperature of 220° C. The split mold having an edge gate was soconstructed, that a pair of flat rectangular dishes of 5 cm×8 cm×0.5 cm,which can be coupled together to form a case having hinge, are moldedsimultaneously.

Inductor was prepared by winding a copper tube having a diameter of 5 mminto whirl at an interval of 5 mm and embedding it in an epoxy resinplate of 2 cm thickness.

This inductor was placed between the two mold halves. The inductor wasactuated by 400 KHz, 6 KW for 15 seconds. After the inductor waswithdrawn from the mold, the injection molding was carried out as inExample 1.

The surface of the thus molded article was the same as that of ordinarymolded product of PS resin without additives, showing no exposure ofiron powder on the outer face. The specific gravity of the moldedarticle was found to be 1.8, exhibiting a solid touch heretofore unseenin conventional PS molded product. GS (60°)% of the molded article was99%.

Comparison Example 1

Using the same injection molding machine, same mold and same resincomposition as in Example 2, injection molding was carried out under thecondition of resin temperature of 240° C., mold temperature of 60° C.,injection duration of 10 seconds, cooling time of 15 seconds, totalinjection cycle of 40 seconds and injection pressure of 50 Kg/cm²without preheating by high-frequency induction. The properties of the soobtained molded article were as given in Table 1.

                  TABLEI                                                          ______________________________________                                        Comparison of Injection Molding with                                          and without High-Frequency Induction Preheating                                                                      Compa-                                            Method of                   rison                                             Examina-              Example                                                                             Example                                Property   tion       Unit       2     1                                      ______________________________________                                        Tensile strength                                                                         JIS K 6871 Kg/cm.sup.2                                                                              1000  1000                                   Elongation JIS K 6871 %          2     2                                      Flexural modulus                                                                         ASTM D 790 Kg/cm.sup.2                                                                              55000 55000                                  Flexural strength                                                                        ASTM D 790 Kg/cm.sup.2                                                                              1300  1300                                   Izod impact                                                                              JIS K 6871 Kg-cm/cm   30    25                                     strength                                                                      (3.2 mm thick,                                                                without notch)                                                                Heat distortion                                                                          JIS K 6871 °C. 104   100                                    temp.                                                                         Gloss      ASTM D 523 Gs (60°) %                                                                        98    45                                     ______________________________________                                    

EXAMPLE 4

It was intended here to make comparison of surface gloss values ofmolded articles at various mold temperatures for various resinsemployed.

A spirit mold made of ultra hard mold metal (NAK metal) with mirrorfinished mold inner surface for molding a flat square piece with aperforation of 1.5 cm φ and having a thickness of 3 mm, as shown in FIG.7, was used, the gate of which was positioned at E in FIG. 7 and was arestrict gate of 4^(W) ×8^(L) ×2^(t) mm. Inductor was prepared bywinding a copper tube having a diameter of 5 mm into a flat whirl at aninterval of 10 mm and embedding it in an epoxy resin plate of athickness of 3 cm. The high-frequency oscillator with continuousvariable output power was used at 7 KHz, 10 KW. For the injectionmolding machine, Toshiba IS 80 (5 oz injection molding machine) wasemployed. The injection molding was carried out by the temporarilyadopted inductor in accordance with the present invention under ordinarycondition using various resin composition. The results were as given inTable 2.

It is seen in Table 2, that injection molded articles having high glossand showing almost no gloss gradient, namely "gloss unequality", can beobtained according to the present invention. In other words, it is seen,that there is considerable difference in the surface gloss between theportions E and F for conventional product, whereas there is no suchdifference for the product according to the present invention inaddition to the high gloss thereof.

                                      TABLE 2                                     __________________________________________________________________________                             Conventional                                                          Injection                                                                             product    Product according                                          molding at                                                                            Max.       to the invention                                               Injec-                                                                            temp.                                                                              GS    Max. GS                                                    Cylin-                                                                            tion                                                                              of mold                                                                            (60°) %                                                                      temp.                                                                              (60°) %                                        der press.                                                                            inner                                                                              por-                                                                             por-                                                                             of mold                                                                            por-                                                                             por-                              Resin            temp.                                                                             (Kg/                                                                              surface                                                                            tion                                                                             tion                                                                             inner                                                                              tion                                                                             tion                              Trade name                                                                            Resin    (°C.)                                                                      cm.sup.2)*                                                                        (°C.)                                                                       E  F  surface                                                                            E  F                                 __________________________________________________________________________    Styron 492                                                                            HIPS     220 35  70   41 32 110  103                                                                              103                               Styron 492                                                                            HIPS     220 50  60   30 25 110  103                                  Stryon 492                                                                            HIPS     220 40  40   20 18 110  103                                                                              103                               Stryon XH 602                                                                         HIPS     220 35  70   51 40 110  102                                                                              102                               Styron 777                                                                    (492-50/683-50)                                                                       MIPS     220 30  70   60 55 110  102                                                                              102                               Styron 777                                                                    (XH 602-30                                                                    /679-70)                                                                              MIPS     220 30  70   85 85 110  102                                                                              101                               Stylac 100                                                                            ABS      230 40  70   70 63 110  100                                                                               99                               Stylac 120                                                                            ABS      230 40  70   89 86 110  100                                                                              100                               Tyril GF                                                                              Glass Fiber                                                           R 140 T Reinforced                                                                             240 45  70   40 39 115  102                                                                              101                                       SAN GF con-                                                                   tent 20 wt. %                                                         Stylac GF                                                                             Glass Fiber                                                           R 240 A Reinforced                                                                             240 45  70   35 30 115   99                                                                               98                                       ABS, GF con-                                                                  tent 20 wt. %                                                         Stylac GF                                                                             Glass Fiber                                                           R 220 A Reinforced                                                                             240 40  70   45 43 115   99                                                                               99                                       ABS, GF con-                                                                  tent 10 wt. %                                                         Xyron GF                                                                              Glass Fiber                                                           G 702 H Reinforced                                                                             290 45  90   30 28 135  102                                                                              102                                       PPE, GF con-                                                                  tent 20 wt. %                                                         Stylac  AB S 60 wt. %                                                         A 4081  Fe.sub.2 O.sub.3 40 wt. %                                                              240 40  70   50 45 120  100                                                                              100                               Xyron 201 V                                                                           PPE      240 45  80   50 45 130  103                                                                              103                               Xyron 500 H                                                                           PPE      290 45  90   45 40 135  103                                                                              102                               HIPS trial                                                                            Rubber con-                                                           product tent 5 wt. %                                                                           220 35  70   45 33 110  103                                                                              103                               HIPS trial                                                                            Rubber con-                                                           product tent 10 wt. %                                                                          220 37  70   35 21 110  102                                                                              102                               HIPS trial                                                                            Rubber con-                                                           product tent 15 wt. %                                                                          220 41  70   20 13 110  101                                                                              100                               Loymer  CaCO.sub.3 40 wt. %                                                   S 3340  fillered pp                                                                            220 50  40   16 17 150   88                                                                               88                               Panlite Glass Fiber                                                           G 1030  Reinforced                                                                             290 90  80   25 20 160  103                                                                              103                                       PC, GF                                                                        cont. 30 wt. %                                                        Amilan  Glass Fiber                                                           CM 10116-30                                                                           Reinforced                                                                             240 50  80   30 25 205   94                                                                               94                                       PA-6, GF                                                                      cont. 30 wt. %                                                        __________________________________________________________________________     *determined based on the norm of "short shot point + 5" Kg/cm.sup.2      

In the molded articles according to the present invention, the faultysurface defects of flow marks, jettings and silver streaks arecompletely avoided and also the weld line is not noticeable for allresins examined.

In the molded products by the conventional practice, a high glossproduct is never obtained even by selecting the resin employed, as canbe seen from Table 2. Flow marks and jettings were found, silver streaksoccurred in molded product containing filler and weld line wasnoticeable. Improvements in the appearance and gloss obtained byalteration of the molding condition, especially of the mold temperatureis limited and does not reach the level according to the presentinvention.

Comparison Example 2

It is contemplated to exemplify the preheating of the metal mold byintroducing a heated fluid into the mold.

The metal mold of Example 4 was used. As the heating fluid, steam of 10Kg/cm² was employed. The metal mold was provided with inlet and outletslits where molten resin cannot flow into but steam at parting planes insuch a manner that an inner surface of the mold could be heated. In theinlet, a check valve was arranged and a trap was installed at theoutlet. In order to allow to maintain the steam pressure within themetal mold, O-rings were employed. It was so difficult to seal up themold against the steam of 10 Kg/cm², that practical industrialapplication of such technique would be impossible, since the metal moldhas a kick out pin and O-ring is not much effective for sealing. Thepreheating of the metal mold was performed with the steam leaking. Thesteam pressure within the mold was kept at 3-4 Kg/cm² for 30 seconds,before it was discharged. The injection molding was carried outsubsequently. The mold temperature at this moment was 120° C. The soobtained molded product was not always fine. Occasionally, speckles onthe molded product were recognized, though the reason therefor was notidentified whether it had been caused from the water rest or thecorrosion inhibitor contained in steam. There was also rest around anejector pin on the molded product, said rest being likely due to moldsweat.

Moreover, a labor safety problem, maintenance of metal mold for, suchas, corrosion prevention etc., problem of durability of O-ring and so onadd thereto, wherefrom it may be concluded that this procedure is farless applicable in industry as compared with the present invention.

EXAMPLE 5

The procedures of Example 1 were followed with the exception thatinstead of the 20% GF-containing SAN resin, a PP resin containing 30% byweight of glass fiber was employed. Results comparable to the SAN resinwere obtained. Gs(60°)% of the molded article was 88%.

EXAMPLE 6

Using a split metal mold, for molding a pair of casing halves for audiocassettes (called cassette halves), an HIPS resin composition wasinjection molded.

The inductor was prepared by winding a copper pipe with diameter of 5 mminto flat whirl at an interval of 5 mm and embedding it in an epoxyresin plate of a thickness of 3 cm. This inductor was placed between themold halves and energized by a high frequency output of 7 KHz, 20 KW for15 seconds. After it was withdrawn from the mold, the injection moldingwas carried out as in Example 1.

The molded article exhibited a complicated configuration having ribs,bosses, perforations and embossed pattern which might have exhibitedflow marks, weld lines and the like if molded by the conventionalinjection molding technique. The molded article showed however, asuperior appearance with no flow mark and no visible weld line butimproved feeling of the embossed pattern. The dimensional accuracy wasthe same as that of injection molded article of prior art. No distortionwas recognized.

By examining the gloss at a flat portion of the molded article, theGs(60°)% values for the article according to the invention and for thearticle of conventional method were found to be 98% and 45%respectively.

EXAMPLE 7

The split metal mold employed was the one shown in FIG. 4, in which theparts A and A' were made of a NAK metal and the surfaces thereofcontacting the injected resin composition were mirror finished. Thismold has a center direct gate and produces a molded article of dish-likeconfiguration having a diameter of 10 cm, a depth of 2 cm and an averagethickness of 3.5 mm. The inductors at the portions B and B' wereprepared each by winding a copper pipe of a diameter of 5 mm into flatwhirl of an interval of 15 mm and embedding it in an epoxy resin plateof a thickness of 15 mm. The mold parts C and C' are made of bronzeplate having a thickness of 3 mm. The mold parts D and D' are made ofmold steel S-45 C.

Using this split mold, a commercially available SAN resin compositioncontaining 20% by weight of glass fiber was injection molded. Theinjection cylinder temperature was adjusted so that the temperature ofthe resin composition was held at 240° C. The inductors were energizedby a high-frequency oscillator of 4 KHz, 8 KW, for 10 seconds.Subsequently, the injection molding was carried out at an injectionpressure of 60 Kg/cm² in an injection period of 10 seconds as in aconventional manner. Then the mold was cooled for 20 seconds before themolded article was withdrawn. The total molding cycle was 45 seconds.

The so obtained molded article showed the same appearance as that of themolded article made of exclusively SAN resin. No silver streak nor theprojection of glass fiber outside the surface of the molded article wasable to recognize. The gloss Gs(60°)% was 102%.

Comparison Example 3

For the sake of comparison, an injection molding was conducted in thesame condition but without employing said high-frequency inductionheating prior to the injection, which resulted in a product withdiscolored appearance showing many silver streaks and having a Gs(60°)value of 45%.

EXAMPLE 8

The apparatus and the procedures were the same as in Example 7, exceptthat the resin employed was changed to HIPS, ABS and PPE and the moldingtemperature was varied so as to match each resin. The molded article ofeach resin had excellent appearance overturning the generally acceptedevaluation of the injection molded articles. Thus, all the moldedarticles obtained exhibited gloss values Gs(60°)% exceeding above 100%regardless of the position on the article and showed no glossirregularities, faulty jettings or so on.

In Table 3, the conditions of injection molding appearance and gloss ofthe molded article for each individual resin composition are recitedtogether with those of comparison Example 4 in which the procedures ofExample 7 were followed except that the preliminary heating of metalmold by high-frequency induction was discarded.

                                      TABLE 3                                     __________________________________________________________________________                  Example 8         Comparison Example 4                                   Oscilla-                                                                           Total             Total                                              Resin                                                                             tion molding           molding                                            temp.                                                                             duration                                                                           cycle                                                                              Appearance   cycle                                                                              Appearance                               Resin                                                                              (°C.)                                                                      (sec.)                                                                             (sec.)                                                                             of product                                                                           Gs(60°) %                                                                    (sec.)                                                                             of product                                                                           Gs(60°)                    __________________________________________________________________________                                                %                                 HIPS 220 10   45   No gloss                                                                             104   35   Gloss  60-70                                                irregular-        irregular-                                                  ity               ity occur                                ABS  240 10   45   No gloss                                                                             104   35   Gloss  85-90                                                irregular-        irregular-                                                  ity               ity occur                                PPE  280 15   50   No gloss                                                                             103   40   Gloss  35-45                                                irregular-        irregular-                                                  ity               ity occur                                SAN-GF                                                                             240 10   45   No gloss                                                                             102   35   Gloss  45-60                             (Example           irregular-        irregular-                               7)                 ity               ity occur                                                                     (rough                                                                        surface)                                 __________________________________________________________________________

We claim:
 1. A method of injection molding thermoplastic resins orthermoplastic resins containing reinforcing material and/or fillerscomprising:heating selective inner surface areas of a metal mold byhigh-frequency induction heating to a temperature above the heatdistortion temperature of the resin; injecting molten resin into themold, thereby flowing the molten resin composition adjacent to the innersurface area of the mold to form a skin layer consisting substantiallyof only the resin; and cooling the mold, opening the mold, and removinga molded article.
 2. A method according to claim 1, wherein the innersurface area heating is performed at a rate of temperature elevation of80° C./min. or more.
 3. A method according to claim 1, wherein thethermoplastic resin contains at least 4% by weight of reinforcingmaterial and/or fillers.